[JENKINS-50777] Expose SCMRevisionAction and associated metadata via Stapler export

[jglick]

JENKINS-50777

Example output with jenkinsci/git-plugin#581 + jenkinsci/github-branch-source-plugin#180 on a master build:

{
  "_class" : "jenkins.scm.api.SCMRevisionAction",
  "revision" : {
    "_class" : "jenkins.plugins.git.AbstractGitSCMSource$SCMRevisionImpl",
    "deterministic" : true,
    "head" : {
      "_class" : "org.jenkinsci.plugins.github_branch_source.BranchSCMHead",
      "name" : "master",
      "origin" : {
        "_class" : "jenkins.scm.api.SCMHeadOrigin$Default"
      }
    },
    "hash" : "c9e8148ede406f8ad89610befbdf51ba4a0ef24a"
  }
}

and on a PR merge build, specifically cloudbeers/PR-demo#20:

{
  "_class" : "jenkins.scm.api.SCMRevisionAction",
  "revision" : {
    "_class" : "org.jenkinsci.plugins.github_branch_source.PullRequestSCMRevision",
    "deterministic" : true,
    "head" : {
      "_class" : "org.jenkinsci.plugins.github_branch_source.PullRequestSCMHead",
      "name" : "PR-20",
      "origin" : {
        "_class" : "jenkins.scm.api.SCMHeadOrigin$Fork",
        "name" : "vivek"
      }
    },
    "merge" : true,
    "target" : {
      "_class" : "jenkins.plugins.git.AbstractGitSCMSource$SCMRevisionImpl",
      "deterministic" : true,
      "head" : {
        "_class" : "org.jenkinsci.plugins.github_branch_source.BranchSCMHead",
        "name" : "master",
        "origin" : {
          "_class" : "jenkins.scm.api.SCMHeadOrigin$Default"
        }
      },
      "hash" : "c9e8148ede406f8ad89610befbdf51ba4a0ef24a"
    },
    "pullHash" : "5d2f682bc7a77f253e9b887536fa08f7ba41af64"
  }
}

Example of extracting important fields from a build URL:

…/job/…/…/api/json?tree=actions[revision[hash,pullHash]]

[jglick]

Note that #35 exported SCMHead.origin yet neglected to add this annotation, causing

org.kohsuke.stapler.export.NotExportableException: class jenkins.scm.api.SCMHeadOrigin$Fork doesn't have @ExportedBean

[svanoort]

Seems okay to me...?